The present invention relates to a capacitor charging circuit and, in particular, to a bootstrap capacitor charging circuit. Even more particularly, the present invention is related to a bootstrap capacitor charging circuit with limited charging current.
Bootstrap capacitors are often employed in electronic circuits to derive a source of power, usually for circuits that require low level current supplies. Often, the bootstrap power supply is derived from a signal present in the electronic circuit such as a drive signal and charge is stored in a bootstrap capacitor for use in powering another portion of the electronic circuit. Often, the voltage is a different voltage than the main supply voltage of the circuit. FIG. 1 shows a typical prior art bootstrap circuit. An integrated circuit IC has a terminal 10 at which a signal, for example, a pulsating drive or control signal may be present. During the times that a pulse is present, current is supplied through a resistor R and a bootstrap diode DBS to a charge storage capacitor or a bootstrap capacitor, CBS. A zener diode DZ may be employed to limit the voltage which can be developed across CBS and therefore provide a regulated bootstrap voltage source VBS for circuits inside the integrated circuit or external to the integrated circuit. In FIG. 1, the diode DZ is shown inside the integrated circuit, although it may be a separate external component. Similarly, the resistor R and diode DBS may be internal to the IC or may be external as shown. Generally, however, the bootstrap capacitor CBS is an external component because of its size.
A problem with the prior art design is that the bootstrap capacitor CBS charges at all times when the pulse is present on the supply line terminal 10. This may be wasteful of power. Depending on the circuits supplied by CBS, it may not be necessary to charge CBS at all times when a pulse is present on line 10.